1. Field of the Invention
This invention relates to a surface mountable electrical device, more particularly to a surface mountable electrical device that can serve as a circuit protection device, such as a fuse.
2. Description of the Related Art
FIG. 1 illustrates a conventional circuit protection device 10 disclosed in U.S. Pat. No. 5,852,397. The device 10 is prepared from a laminate having an array of drilled apertures. The laminate is cut into a plurality of the devices along intersecting cutting lines in two directions. The apertures are located at positions where the cutting lines intersect. The device 10 includes a planar PTC (positive temperature coefficient) resistive element 11 having opposite first and second surfaces 111, 112, and four apertures 113 of quarter circular cross-section that run between the first and second surfaces 111, 112 at four corners of the PTC resistive element 11, first and second electrode layers 12, 13 respectively covering portions of the first and second surfaces 111, 112, third and fourth electrode layers 14, 15 respectively covering other portions of the first and second surfaces 111, 112 and spaced apart from the first and second electrode layers 12, 13, a first plating layer 16 having first and second portions 161, 162 respective covering the first and fourth electrode layers 12, 15 and two spaced apart first transverse portions 163 extending between the first and second portions 161, 162 within two of the apertures 113 that are adjacent to the fourth electrode layer 15, and a second conductive layer 17 having third and fourth portions 171, 172 respectively covering the second and third electrode layers 13, 14 and two second transverse portions 173 extending between the third and fourth portions 171, 172 within another two of the apertures 113 that are adjacent to the third electrode layer 14. The first and second transverse portions 163, 173 have a quarter circular cross-section corresponding to those of the apertures 113. Aside from being formed at the corners of the PTC resistive element 11 and having quarter cross-sections, the aforesaid apertures 113 can be formed at any other positions in the PTC resistive element 11, and can have a closed cross-section, such as a circular, an oval, or a rectangular shape, or a reentrant cross-section that denotes an open cross-section which has a depth of at least 0.15 times the maximum width thereof and which has at least one part where the opposite edges of the cross section are parallel to each other in order to ensure that the first and second transverse portions 163, 173 of the first and second plating layers 16, 17 will not be damaged or dislodged during installation or use of the device 10. However, since the maximum width of each aperture 113 is very small, which is normally designed to be within a range of 0.2 to 0.5 mm, there is a tendency for the first and second transverse portions 163, 173 of the first and second plating layers 16, 17 to be damaged or dislodged during the process of cutting the laminate to form the devices 10 with apertures of reentrant cross-sections, and there is a need for a relatively high accuracy to maintain the aforementioned cutting lines.
FIGS. 2 and 3 illustrate another conventional circuit protection device 8 disclosed in U.S. Pat. No. 5,884,391. The device 8 is prepared from a laminar sheet 9 having an array of strips 90 that includes a plurality of repeated units 92 that are to be processed into the devices 8, and an array of elongated slits 91 at intervals of the strips 90. The device 8 includes a PTC resistive element 80 with opposite first and second surfaces 801, 802, first and second electrode layers 81, 82 respectively formed on portions of the first and second surfaces 801, 802, third and fourth electrode layers 83, 84 respectively formed on other portions of the first and second surfaces 801, 802, first and second insulating layers 85, 86 respectively formed on portions of the first and third electrode layers 81, 83 and portions of the second and fourth electrode layers 82, 84, U-shaped first and second conductive layers 87, 88 respectively formed on and interconnecting portions of the first and fourth electrode layers 81, 84 and portions of the second and third electrode layers 82, 83, and U-shaped first and second end terminations 871, 881 respectively superimposed on the first and second conductive layers 87, 88. Each of the U-shaped first and second conductive layers 87, 88 has two lateral segments (871, 881) that extend in lateral directions along the first and second surfaces 801, 802, and a transverse segment (872, 882) that extends in a transverse direction relative to the first and second surfaces 801, 802 at one side of the PTC resistive element 80 that faces a respective slit 91. Since the transverse segment (871, 881) of each of the first and second conductive layers (87, 88) is only formed on one side of the PTC resistive element 80, the device 8 still encounters the same problem of the previous device 10 disclosed in U.S. Pat. No. 5,852,397, and needs a relatively high accuracy when cutting the units of the laminar sheet 9 into the devices 8.